Radioactive sources are frequently used in measurement devices.
One example of use of a radioactive source is a Logging While Drilling (LWD) application in which a sonde is lowered in a well being drilled. The sonde typically contains a radioactive source and one or more suitable detectors. The sonde allows to gather information about the earth formation being drilled and surrounding the well. The sonde progresses in the well while drilling, and it thus becomes possible to measure some parameters of the earth's formations.
The radioactive source may for example be a chemical radioactive source and be transported in a carrying shield device. Prior to the LWD measurement, the radioactive source needs to be transferred from the carrying shield device into a location of an LWD collar. After LWD measurement, the radioactive source is extracted from its location in the LWD collar back into the carrying shield device.
The carrying shield device is typically made from an appropriate material that prevents manipulating personnel from being exposed to the radioactivity. In case the personnel does get exposed to radioactivity, the radioactive dose absorbed by personnel during exposure is a function of the time spent and the inverse of the square of the distance separating the personnel from the source. In other words, if the distance between the personnel and the source is reduced by half, the absorbed dose may be increased fourfold. For this reason the distance between the source and the personnel must at all times be maintained at the highest possible value.
The manipulating of the source is a critical process where contradictory requirements of keeping the personnel at a safe distance and preventing the loss of the source must be met at the same time.
GB 2276898 A (TELECO OILFIELD SERVICES NC) Dec. 10, 1994 discloses an example of container housing for introducing and mounting a radioactive source within a passage in a borehole logging tool. The contained has at one end a threaded extension for mounting in the logging tool. The container has at its other end a bayonet for engagement with the bayonet connection of a handling tool for inserting the container into the passage.
In order to prevent the loss of the source, it is known to implement dual attachment systems for the source in the carrying shield device and in the logging tool. It is further known to use source handling systems comprising redundant source attachment systems. In such redundant systems, if one system fails to retain the source, the other may still be effectively attaching the source.
One example of a source handling system comprising a redundant attachment system uses a set of fingers and a wire spring clip. The set of fingers is used as a first means for holding the source. The set of fingers is mounted at one source proximate extremity of an elongated handling tool. The fingers typically close around a conical head of the source. The fingers are closed by screwing a handle at an extremity of the handling tool opposite from the source side extremity. Next, the wire spring clip is used as a second means for attaching the source to the handling tool. The wire spring clip is attached to a piece of chain. One extremity of the chain is permanently attached to the source proximate extremity of the handling tool. In order to attach the source with the wire spring clip, the source comprises an appropriate hole, and the clip is pushed into the hole of the source. The attachment of the wire spring clip to the source requires that the personnel brings its hands within a few centimeters of the source.
In the described example of redundant attachment system, it appears that the wire spring clip is frequently not used. One reason for not using the clip is personnel's concern to be exposed to radioactivity. Another reason for not using the clip is its relative unreliability to hold the source should it escape from a grip of the fingers.